1. Field of the Invention
The present invention relates in general to a metal bump. In particular, the present invention relates to a profile-design rule of a metal bump for reducing a diagonal distance between two adjacent metal bumps positioned at adjacent rows.
2. Description of the Related Art
The attachment of a bared die to a glass panel (called COG: chip on glass) is one advanced application for electrically connecting integrated circuits (ICs) achieving lighter weight, smaller size, lower cost and less power consumption demanded in various display products. The quality and reliability of the liquid crystal display (LCD) module depends on the way in which the driver IC is attached to the glass panel. Anisotropic conductive film (ACF) is the most popular material for attaching the chip to the glass panel. ACF is an adhesive film consisting of dispersed, microscopic, electrically conductive particles 3-15 xcexcm in diameter and an insulating adhesive film 15-35 xcexcm thick. Various kinds of conductive particles, such as carbon fiber, metal (Ni, solder), and metal (Ni/Au)-coated plastic balls have been proposed, and the uniformity of the conductive particles distribution is considered an influence on the electrical property and reliability of ACF. Also, various types of adhesive materials, such as thermoplastic, thermosetting, and mixed thermoplastic and thermosetting materials have been proposed. In general, ACF is classified into two types. One has conductive particles 5 xcexcm in diameter covered with a very thin insulating layer, wherein the thin insulating layer is broken when the particles are deformed, the bared conductive particles serving as a bridge for electrically connecting the metal bump on the chip and the bonding pad on the glass panel. However, the breaking of the conductive particles during the fabricating process cannot be ensured; therefore, there is no guarantee of effective contact between the metal bump and the bonding pad. The other type of ACF is a double-layer type, which consists of one layer filled with conductive particles 3 xcexcm in diameter and the other layer with no conductive particles, so that the functions of conduction and adhesion are separated. This can ensure the effective contact between the metal bump and the bonding pad. Nevertheless, when too many conductive particles exist in the space between two adjacent metal bumps, a lateral connection between the two adjacent metal bumps is easily formed, resulting in an electrical short.
FIG. 1A to FIG. 1C are schematic cross-sectional diagrams of a method of connecting a chip 14 and a glass substrate 10 according to the prior art. The glass substrate 10 of the LCD module comprises a first area for disposing an array of thin film transistors (TFTs), a second area for disposing data IC chips or scan IC chips 14, and a plurality of bonding pads 12 are formed on the second areas. The chip 14 has a plurality of metal pads 16 and a plurality of metal bumps 18, wherein each metal bump 18 is patterned on each metal pad 16 and corresponds in position to each bonding pad 12. In the prior method of connecting the chip 14 and the glass substrate 10, as shown in FIG. 1A, an ACF 20 is attached to the surface of the glass substrate 10 to cover the bonding pad 12. Then, the surface of the chip 14 is downwardly placed on the predetermined area of the glass substrate 10, wherein each metal bump 18 corresponds to each bonding pad 12 of the glass substrate 10. As shown in FIG. 1B, by means of the adhesion of the ACF 20 and the downwardly exerted pressure, the chip 14 is tightly attached to the glass substrate 10. Next, a thermal process is performed to cure the ACF 20. The conductive particles 22 sandwiched between the top of the metal bump 18 and the surface of the bonding pad 12 now serve as an electrically connecting bridge, as shown in FIG. 1C. However, the distribution of the conductive particles 22 cannot be controlled in processing, and thereby many conductive particles 22 that exist between adjacent metal bumps 18 may laterally connect with each other to cause electrical shorts.
FIG. 2A shows a top view of the layout of the metal bumps 18 according to the prior art. For providing great output terminals and avoiding electrical shorts between metal bumps 18, the metal bumps 18 are generally arranged in two rows. In each row, each metal bump 18 with a transverse width W2 is spaced out a transverse distance W1 apart from each other, and the tops of the metal bumps 18 are leveled off. For example, in a first row, a first metal bump 181 and a second metal bump 182 are adjacent and apart from the transverse distance W1. In a second row, a third metal bump 183 is disposed between the first metal bump 181 and the second bump 182. Therefore, the three centers of the three bumps 181, 182, 183 respectively are arranged as a triangle. Notice that the transverse distance W1 is equal to the transverse width W2, and a lengthwise distance L between the first row and the second row is smaller than the transverse distance W1. However, since the metal bump 18 is shaped into a square or rectangular profile, the shortest distance is found between a point A of the first metal bump 181 and a point B of the third metal bump 183. A lateral connection between the first metal bump 181 and the third metal bump 183 is easily formed by the conductive particles to result in an electrical short. Similarly, a lateral connection is easily formed between a point C of the second metal bump 182 and a point D of the third metal bump 183.
In addition, an electrical short is easily caused by an alignment error between the metal bump 18 and the bonding pad 12. Please refer to FIG. 2B, which shows a top view of the metal bump 18 and the bonding pad 12 according to the prior art. In general, the profile of the bonding pad 12 is square or rectangular according to the profile of the metal bump 18, and the surface area of the bonding pad 12 is larger than the top area of the metal bump 18. Thereby, a tolerance limitation of an alignment error in COG technique depends on the shorted distance d between the first bonding pad 121 and the third bonding pad 123. When the chip 14 is inaccurately attached to the glass substrate 10, the up-left corner or the up-right corner of the third bonding pad 123 is easily contacted with the point A of the first metal bump 181 or the point C of the second metal bump 182. Similarly, the down-right corner of the first bonding pad 121 or the down-left corner of the second bonding pad 122 is easily contacted with the point B or D of the third metal bump 183.
From the above-described disadvantages, the square or rectangular profile of the metal bump 18 reduces the diagonal distance between two adjacent metal bumps 18 positioned at adjacent rows. This causes electrical shorts and limits the tolerance of alignment error in COG technique, resulting in lowering quality and reliability of LCD modules.
An object of the present invention is to provide a cylindrical bump to increase the diagonal distance between metal bumps positioned in adjacent rows.
Another object of the present invention is to provide a polygonal bump to increase the diagonal distance between metal bumps positioned in adjacent rows.
The metal bumps of the present invention include at least a first metal bump having at least one curved face, at least a second metal bump having at least one curved face, and at least a third metal bump having at least a first curved face and a second curved face. The three centers of the first metal bump, the second metal bump and the third metal bump are arranged as a triangle. The first curved face of the third metal bump is adjacent to the curved face of the first metal bump. The second curved face of the third metal bump is adjacent to the curved face of the second metal bump.
Another embodiment of the present invention includes at least a first metal bump having at least one faceted face, at least a second metal bump having at least one faceted face, and at least a third metal bump having at least a first faceted face and a second faceted face. The three centers of the first metal bump, the second metal bump and the third metal bump are arranged as a triangle. The first faceted face of the third metal bump is adjacent to the faceted face of the first metal bump. The second faceted face of the third metal bump is adjacent to the faceted face of the second metal bump.
It is an advantage of the present invention that these metal bumps avoid a lateral electrical connection, thus improving the tolerance limitation of alignment errors in COG technique.
This and other objective of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various figures and drawings.